# Mini Glossary (with references)

**Light cone.** The boundary between events that can be causally connected at ≤ c and those that cannot (Carroll, 2004; Hawking & Ellis, 1973).  
**Worldline.** The path of an object through spacetime, one event per moment of its existence (Hartle, 2003).  
**Proper time.** Time accumulated along a specific worldline; invariant for that worldline (Taylor & Wheeler, 1992; Rindler, 2006).  
**Geodesic.** Natural free-fall path determined by spacetime geometry (Wald, 1984).  
**Spacelike slice (Σ).** A 3D cut through 4D spacetime selecting frame-dependent simultaneity (Hawking & Ellis, 1973).

**References**  
Carroll, S. M. (2004). *Spacetime and Geometry*. Addison-Wesley.  
Hartle, J. B. (2003). *Gravity: An Introduction to Einstein’s General Relativity*. Addison-Wesley.  
Hawking, S. W., & Ellis, G. F. R. (1973). *The Large Scale Structure of Space-Time*. Cambridge University Press.  
Rindler, W. (2006). *Relativity: Special, General and Cosmological* (2nd ed.). Oxford University Press.  
Taylor, E. F., & Wheeler, J. A. (1992). *Spacetime Physics* (2nd ed.). W. H. Freeman.  
Wald, R. M. (1984). *General Relativity*. University of Chicago Press.
